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                <h2 id="「力扣」第-316-题：去除重复字母（困难）"><a href="#「力扣」第-316-题：去除重复字母（困难）" class="headerlink" title="「力扣」第 316 题：去除重复字母（困难）"></a>「力扣」第 316 题：去除重复字母（困难）</h2><p>我写的题解地址：</p>
<blockquote>
<p>给定一个仅包含小写字母的字符串，去除字符串中重复的字母，使得每个字母只出现一次。需保证返回结果的字典序最小（要求不能打乱其他字符的相对位置）。</p>
<p>示例 1:</p>
<pre><code>输入: &quot;bcabc&quot;
输出: &quot;abc&quot;</code></pre><p>示例 2:</p>
<pre><code>输入: &quot;cbacdcbc&quot;
输出: &quot;acdb&quot;</code></pre></blockquote>
<p>来源：力扣（LeetCode）<br>链接：<a href="https://leetcode-cn.com/problems/remove-duplicate-letters" target="_blank" rel="noopener">https://leetcode-cn.com/problems/remove-duplicate-letters</a><br>著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。</p>
<p>同「力扣」第 1081 题：<a href="https://leetcode-cn.com/problems/smallest-subsequence-of-distinct-characters/" target="_blank" rel="noopener">1081. 不同字符的最小子序列</a>。</p>
<h3 id="方法：使用栈"><a href="#方法：使用栈" class="headerlink" title="方法：使用栈"></a>方法：使用栈</h3><blockquote>
<p>1、栈里永远存放的是目前看到的字典序最小的子序列；</p>
<p>2、什么时候栈顶出栈？（1）非空；（2）字典序：栈顶 &gt; 新来的字符；（3）栈顶元素在以后还会看到。</p>
<p>此时，可以得到一个字典序更小的子序列。</p>
<p>3、如果新来的字符，栈里没有，并且以后也不会看到，就必须添加；</p>
<p>如果新来的字符，栈里有，就不用管了，因为如果抛弃了栈里的，得到的字典序更大。</p>
</blockquote>
<p>![image-20191207212106043](/Users/liwei/Library/Application Support/typora-user-images/image-20191207212106043.png)</p>
<p>Java 代码：</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">import</span> java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>HashMap<span class="token punctuation">;</span>
<span class="token keyword">import</span> java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>HashSet<span class="token punctuation">;</span>
<span class="token keyword">import</span> java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>Map<span class="token punctuation">;</span>
<span class="token keyword">import</span> java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>Set<span class="token punctuation">;</span>
<span class="token keyword">import</span> java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>Stack<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Solution</span> <span class="token punctuation">{</span>

    <span class="token keyword">public</span> String <span class="token function">removeDuplicateLetters</span><span class="token punctuation">(</span>String s<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">int</span> len <span class="token operator">=</span> s<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment" spellcheck="true">// 预处理，我把一个字符出现的最后一个位置记录下来</span>
        Map<span class="token operator">&lt;</span>Character<span class="token punctuation">,</span> Integer<span class="token operator">></span> map <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token operator">&lt;</span><span class="token operator">></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> len<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            map<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span>s<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">,</span> i<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>

        <span class="token comment" spellcheck="true">// 保存已经出现过的</span>
        Set<span class="token operator">&lt;</span>Character<span class="token operator">></span> set <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashSet</span><span class="token operator">&lt;</span><span class="token operator">></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        Stack<span class="token operator">&lt;</span>Character<span class="token operator">></span> stack <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Stack</span><span class="token operator">&lt;</span><span class="token operator">></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> len<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

            Character curChar <span class="token operator">=</span> s<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>set<span class="token punctuation">.</span><span class="token function">contains</span><span class="token punctuation">(</span>curChar<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                <span class="token keyword">continue</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>

            <span class="token comment" spellcheck="true">// 注意：这里条件判断语句很长，map.get(stack.peek()) 不要写成了 map.get(curChar)</span>
            <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">!</span>stack<span class="token punctuation">.</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> map<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>stack<span class="token punctuation">.</span><span class="token function">peek</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">></span> i <span class="token operator">&amp;&amp;</span> curChar <span class="token operator">&lt;</span> stack<span class="token punctuation">.</span><span class="token function">peek</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                <span class="token comment" spellcheck="true">// 出栈的同时，也要从哈希表中移除</span>
                set<span class="token punctuation">.</span><span class="token function">remove</span><span class="token punctuation">(</span>stack<span class="token punctuation">.</span><span class="token function">pop</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>

            stack<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>curChar<span class="token punctuation">)</span><span class="token punctuation">;</span>
            set<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span>curChar<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>

        StringBuilder stringBuilder <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">!</span>stack<span class="token punctuation">.</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            stringBuilder<span class="token punctuation">.</span><span class="token function">insert</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> stack<span class="token punctuation">.</span><span class="token function">pop</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>

        <span class="token keyword">return</span> stringBuilder<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        Solution solution <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Solution</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        String res <span class="token operator">=</span> solution<span class="token punctuation">.</span><span class="token function">removeDuplicateLetters</span><span class="token punctuation">(</span><span class="token string">"cbacdcbc"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>res<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h2 id="参考资料"><a href="#参考资料" class="headerlink" title="参考资料"></a>参考资料</h2><p>1、<a href="https://leetcode-cn.com/problems/smallest-subsequence-of-distinct-characters/solution/mei-you-qi-ji-yin-qiao-zhi-you-chun-cui-de-tan-xin/" target="_blank" rel="noopener">https://leetcode-cn.com/problems/smallest-subsequence-of-distinct-characters/solution/mei-you-qi-ji-yin-qiao-zhi-you-chun-cui-de-tan-xin/</a></p>
<p>Java 代码：</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">import</span> java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>Stack<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Solution</span> <span class="token punctuation">{</span>

    <span class="token comment" spellcheck="true">// 贪心算法</span>

    <span class="token keyword">public</span> String <span class="token function">removeDuplicateLetters</span><span class="token punctuation">(</span>String s<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment" spellcheck="true">// 计算 26 字母数量</span>
        <span class="token keyword">int</span><span class="token punctuation">[</span><span class="token punctuation">]</span> charsCount <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">int</span><span class="token punctuation">[</span><span class="token number">26</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
        <span class="token comment" spellcheck="true">// 标记字母是否已经入栈</span>
        <span class="token keyword">boolean</span><span class="token punctuation">[</span><span class="token punctuation">]</span> visited <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">boolean</span><span class="token punctuation">[</span><span class="token number">26</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
        <span class="token keyword">int</span> len <span class="token operator">=</span> s<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">char</span><span class="token punctuation">[</span><span class="token punctuation">]</span> sChars <span class="token operator">=</span> s<span class="token punctuation">.</span><span class="token function">toCharArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">char</span> c <span class="token operator">:</span> sChars<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            charsCount<span class="token punctuation">[</span>c <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span><span class="token operator">++</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        Stack<span class="token operator">&lt;</span>Character<span class="token operator">></span> stack <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Stack</span><span class="token operator">&lt;</span><span class="token operator">></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment" spellcheck="true">// 最终字符的长度</span>
        <span class="token keyword">int</span> index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> count <span class="token operator">:</span> charsCount<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>count <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                index<span class="token operator">++</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>

        <span class="token keyword">char</span><span class="token punctuation">[</span><span class="token punctuation">]</span> res <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">char</span><span class="token punctuation">[</span>index<span class="token punctuation">]</span><span class="token punctuation">;</span>
        <span class="token comment" spellcheck="true">// len 是字符串的长度</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> len<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">char</span> c <span class="token operator">=</span> s<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment" spellcheck="true">// 有小字符的且满足其前面的字符在小字符后还有同样字符的，则出栈</span>
            <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">!</span>stack<span class="token punctuation">.</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> c <span class="token operator">&lt;</span> stack<span class="token punctuation">.</span><span class="token function">peek</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> charsCount<span class="token punctuation">[</span>stack<span class="token punctuation">.</span><span class="token function">peek</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span> <span class="token operator">></span> <span class="token number">1</span> <span class="token operator">&amp;&amp;</span> <span class="token operator">!</span>visited<span class="token punctuation">[</span>c <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                Character pop <span class="token operator">=</span> stack<span class="token punctuation">.</span><span class="token function">pop</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                visited<span class="token punctuation">[</span>pop <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
                charsCount<span class="token punctuation">[</span>pop <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span><span class="token operator">--</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>visited<span class="token punctuation">[</span>c <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                <span class="token comment" spellcheck="true">//重复的字符根据游标往后移动，数量减一</span>
                charsCount<span class="token punctuation">[</span>c <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span><span class="token operator">--</span><span class="token punctuation">;</span>
                <span class="token keyword">continue</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
            stack<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span><span class="token punctuation">;</span>
            visited<span class="token punctuation">[</span>c <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>

        <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">!</span>stack<span class="token punctuation">.</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            res<span class="token punctuation">[</span><span class="token operator">--</span>index<span class="token punctuation">]</span> <span class="token operator">=</span> stack<span class="token punctuation">.</span><span class="token function">pop</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        <span class="token keyword">return</span> String<span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span>res<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>Java 代码：</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">import</span> java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>Stack<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Solution</span> <span class="token punctuation">{</span>

    <span class="token keyword">public</span> String <span class="token function">removeDuplicateLetters</span><span class="token punctuation">(</span>String s<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">int</span> len <span class="token operator">=</span> s<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        Stack<span class="token operator">&lt;</span>Character<span class="token operator">></span> stack <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Stack</span><span class="token operator">&lt;</span><span class="token operator">></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> len<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>stack<span class="token punctuation">.</span><span class="token function">contains</span><span class="token punctuation">(</span>s<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                <span class="token keyword">continue</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
            <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">!</span>stack<span class="token punctuation">.</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> s<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span> <span class="token operator">&lt;</span> stack<span class="token punctuation">.</span><span class="token function">peek</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> s<span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span>stack<span class="token punctuation">.</span><span class="token function">peek</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> i<span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                stack<span class="token punctuation">.</span><span class="token function">pop</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
            stack<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span>s<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>

        StringBuilder sb <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span>Character c <span class="token operator">:</span> stack<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            sb<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        <span class="token keyword">return</span> sb<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h1 id="LeetCode-第-316-题：去除重复字母（困难）"><a href="#LeetCode-第-316-题：去除重复字母（困难）" class="headerlink" title="LeetCode 第 316 题：去除重复字母（困难）"></a>LeetCode 第 316 题：去除重复字母（困难）</h1><p>传送门：<a href="https://leetcode-cn.com/problems/remove-duplicate-letters/" target="_blank" rel="noopener">LeetCode 第 316 题：去除重复字母</a>。</p>
<p>同  <a href="https://leetcode-cn.com/problems/smallest-subsequence-of-distinct-characters/" target="_blank" rel="noopener">LeetCode 第 1081 题：不同字符的最小子序列</a>。</p>
<blockquote>
<p>给定一个仅包含小写字母的字符串，去除字符串中重复的字母，使得每个字母只出现一次。需保证返回结果的字典序最小（要求不能打乱其他字符的相对位置）。</p>
</blockquote>
<p>示例 1:</p>
<pre><code>输入: &quot;bcabc&quot;
输出: &quot;abc&quot;</code></pre><p>示例 2:</p>
<pre><code>输入: &quot;cbacdcbc&quot;
输出: &quot;acdb&quot;</code></pre><p>来源：力扣（LeetCode）<br>链接：<a href="https://leetcode-cn.com/problems/remove-duplicate-letters" target="_blank" rel="noopener">https://leetcode-cn.com/problems/remove-duplicate-letters</a><br>著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。</p>
<p><strong>思路分析：</strong></p>
<p>首先解释一下字典序是什么。</p>
<p><strong>字典序</strong></p>
<p>字典序是指从前到后比较两个字符串大小的方法。</p>
<ul>
<li>首先比较第 1 个字符，如果不同则第 1 个字符较小的字符串更小；</li>
<li>如果相同则继续比较第 2 个字符 …… 如此继续，比较整个字符串的大小。</li>
</ul>
<p>整理题目要求：</p>
<p>1、去除字符串中重复的字母，使得每个字母只出现一次；</p>
<p>2、保证返回结果的字典序最小；</p>
<p>3、不能打乱其他字符的相对位置。</p>
<p>观察示例 1：<code>bcabc</code>。</p>
<ul>
<li>如果某个字符在字符串中只出现一次，那么这个字符串必须被选取，这里 <code>a</code> 必须被选取；</li>
<li>字符 <code>b</code> 出现了两次，显然选择 <code>a</code>后面的那个，因为字典序 <code>ab</code> 在 <code>ba</code> 前面。同理，有两个相同的字符 <code>c</code> ，我们选择后一个。因此，输出就是 <code>abc</code>。</li>
</ul>
<p>再观察示例 2：<code>cbacdcbc</code>。</p>
<ul>
<li>有 4 个字符：<code>a</code>、<code>b</code>、<code>c</code>、<code>d</code>。其中 <code>a</code> 和 <code>d</code> 只出现一次，必须被选取；</li>
<li><code>b</code> 出现 2 次，一个在 <code>a</code> 前面，一个在 <code>a</code> 后面，显然保留在 <code>a</code> 后面的；</li>
<li><code>c</code> 出现 4 次，我们把几种可能都列出来一下：</li>
</ul>
<p>情况 1：<code>cadb</code><br>情况 2：<code>acdb</code>（字典序最小）<br>情况 3：<code>adcb</code><br>情况 4：<code>adbc</code></p>
<p>一种最理想的情况是：<code>abcd</code>，在遍历的时候，遇到的字符串的 ASCII 值逐渐增大。下面我们就思考，当遍历到的字符的 ASCII 值减少的时候，应该如何处理。</p>
<p>还看示例 1：已经遍历读到了 <code>bc</code>，即将读到的 <code>a</code> 比 <code>c</code> 的ASCII 值小。这时候看字符 <code>c</code> 在后面还会不会出现，<code>c</code> 会出现，那么 <code>a</code> 之前的 <code>c</code> 我们舍弃。同理，我们舍弃之前的 <code>b</code>，因为 <code>b</code> 在将来还会出现，构成的字典序更小。</p>
<p>到此为止，应该想到我们需要借助栈帮助我们完成这题。</p>
<p>然后，根据这个思路，走一下示例 2：<code>cbacdcbc</code>。</p>
<p>第 1 步：读到 <code>c</code>，入栈，此时栈中元素 <code>[c]</code>；</p>
<p>第 2 步：读到 <code>b</code>，<code>b</code> 比之前 <code>a</code> 小，<code>c</code> 在以后还会出现，因此 <code>c</code> 出栈，<code>b</code> 入栈，此时栈中元素 <code>[b]</code>；</p>
<p>第 3 步：读到 <code>a</code>，<code>a</code> 比之前 <code>b</code> 小，<code>b</code> 在以后还会出现，因此 <code>b</code> 出栈，<code>a</code> 入栈，此时栈中元素 <code>[a]</code>；</p>
<p>第 4 步：读到 <code>c</code>，<code>c</code> 比之前 <code>a</code> 大，直接让 <code>c</code> 入栈，此时栈中元素 <code>[a, c]</code>；</p>
<p>第 5 步：读到 <code>d</code>，<code>d</code> 比之前 <code>d</code> 大，直接让 <code>d</code> 入栈，此时栈中元素 <code>[a, c, d]</code>；</p>
<p>第 6 步：读到 <code>c</code>，这里要注意：<strong>此时栈中已经有 <code>c</code> 了，此时栈中元素构成的字符顺序就是最小的字典序，不可能舍弃之前的 c，而用现在的读到的 <code>c</code></strong>，因此这个 <code>c</code> 不需要，直接跳过；</p>
<p>第 7 步：读到 <code>b</code>，<code>b</code> 比之前的 <code>c</code> 小，但是，后面不会再出现 <code>b</code> 了，因此 <code>b</code> 就应该放在这个位置，因此让 <code>b</code> 入栈，此时栈中元素 <code>[a, c, d, b]</code>；</p>
<p>第 8 步：读到 <code>c</code>，同第 6 步，这个 <code>c</code> 我们不需要。</p>
<p>于是，我们可以设计如下算法：</p>
<blockquote>
<p>1、遍历字符串里的字符，如果读到的字符的 ASCII 值是升序，依次存到一个栈中；<br>2、如果读到的字符在栈中已经存在，这个字符我们不需要；<br>3、如果读到的 ASCII 值比栈顶元素严格小，看看栈顶元素在后面是否还会出现，如果还会出现，则舍弃栈顶元素，而选择后出现的那个字符，这样得到的字典序更小。</p>
</blockquote>
<p>因为需要判断读到的字符在栈中是否已经存在，因此可以使用哈希表，又因为题目中说，字符只会出现小写字母，用一个布尔数组也是可以的，注意在出栈入栈的时候，需要同步更新一下这个布尔数组。<br>又因为要判断栈顶元素在后面是否会被遍历到，因此我们需要先遍历一次字符，存一下这个字符最后出现的位置，就能判断栈顶元素在后面是否会被遍历到。</p>
<p><strong>参考代码 1</strong>：</p>
<pre class="line-numbers language-Java"><code class="language-Java">import java.util.Stack;

public class Solution {

    public String removeDuplicateLetters(String s) {
        int len = s.length();
        // 特判
        if (len < 2) {
            return s;
        }

        // 记录是否在已经得到的字符串中
        boolean[] set = new boolean[26];

        // 记录每个字符出现的最后一个位置
        int[] indexes = new int[26];
        for (int i = 0; i < len; i++) {
            indexes[s.charAt(i) - 'a'] = i;
        }

        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < len; i++) {
            char currentChar = s.charAt(i);
            if (set[currentChar - 'a']) {
                continue;
            }

            while (!stack.empty() && stack.peek() > currentChar && indexes[stack.peek() - 'a'] >= i) {
                char top = stack.pop();
                set[top - 'a'] = false;
            }

            stack.push(currentChar);
            set[currentChar - 'a'] = true;
        }

        StringBuilder stringBuilder = new StringBuilder();
        while (!stack.empty()) {
            stringBuilder.insert(0, stack.pop());
        }
        return stringBuilder.toString();
    }
}<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<pre class="line-numbers language-Python"><code class="language-Python"><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>
<p>Java 代码：</p>
<pre class="line-numbers language-Java"><code class="language-Java">import java.util.Stack;

/**
 * @author liwei
 * @date 2019/6/27 10:26 AM
 */
public class Solution2 {

    public String removeDuplicateLetters(String s) {
        int len = s.length();
        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < len; i++) {
            if (stack.contains(s.charAt(i))) {
                continue;
            }
            while (!stack.isEmpty() && s.charAt(i) < stack.peek() && s.indexOf(stack.peek(), i) != -1) {
                stack.pop();
            }
            stack.add(s.charAt(i));
        }

        StringBuilder sb = new StringBuilder();
        for (Character c : stack) {
            sb.append(c);
        }
        return sb.toString();
    }
}<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>Java 代码：</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token keyword">import</span> java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>Stack<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Solution</span> <span class="token punctuation">{</span>

    <span class="token comment" spellcheck="true">// 贪心算法</span>

    <span class="token keyword">public</span> String <span class="token function">removeDuplicateLetters</span><span class="token punctuation">(</span>String s<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment" spellcheck="true">// 计算 26 字母数量</span>
        <span class="token keyword">int</span><span class="token punctuation">[</span><span class="token punctuation">]</span> charsCount <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">int</span><span class="token punctuation">[</span><span class="token number">26</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
        <span class="token comment" spellcheck="true">// 标记字母是否已经入栈</span>
        <span class="token keyword">boolean</span><span class="token punctuation">[</span><span class="token punctuation">]</span> visited <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">boolean</span><span class="token punctuation">[</span><span class="token number">26</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
        <span class="token keyword">int</span> len <span class="token operator">=</span> s<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">char</span><span class="token punctuation">[</span><span class="token punctuation">]</span> sChars <span class="token operator">=</span> s<span class="token punctuation">.</span><span class="token function">toCharArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">char</span> c <span class="token operator">:</span> sChars<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            charsCount<span class="token punctuation">[</span>c <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span><span class="token operator">++</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        Stack<span class="token operator">&lt;</span>Character<span class="token operator">></span> stack <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Stack</span><span class="token operator">&lt;</span><span class="token operator">></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment" spellcheck="true">// 最终字符的长度</span>
        <span class="token keyword">int</span> index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> count <span class="token operator">:</span> charsCount<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>count <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                index<span class="token operator">++</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>

        <span class="token keyword">char</span><span class="token punctuation">[</span><span class="token punctuation">]</span> res <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">char</span><span class="token punctuation">[</span>index<span class="token punctuation">]</span><span class="token punctuation">;</span>
        <span class="token comment" spellcheck="true">// len 是字符串的长度</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> len<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">char</span> c <span class="token operator">=</span> s<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment" spellcheck="true">// 有小字符的且满足其前面的字符在小字符后还有同样字符的，则出栈</span>
            <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">!</span>stack<span class="token punctuation">.</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> c <span class="token operator">&lt;</span> stack<span class="token punctuation">.</span><span class="token function">peek</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> charsCount<span class="token punctuation">[</span>stack<span class="token punctuation">.</span><span class="token function">peek</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span> <span class="token operator">></span> <span class="token number">1</span> <span class="token operator">&amp;&amp;</span> <span class="token operator">!</span>visited<span class="token punctuation">[</span>c <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                Character pop <span class="token operator">=</span> stack<span class="token punctuation">.</span><span class="token function">pop</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                visited<span class="token punctuation">[</span>pop <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
                charsCount<span class="token punctuation">[</span>pop <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span><span class="token operator">--</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>visited<span class="token punctuation">[</span>c <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                <span class="token comment" spellcheck="true">//重复的字符根据游标往后移动，数量减一</span>
                charsCount<span class="token punctuation">[</span>c <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span><span class="token operator">--</span><span class="token punctuation">;</span>
                <span class="token keyword">continue</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
            stack<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span><span class="token punctuation">;</span>
            visited<span class="token punctuation">[</span>c <span class="token operator">-</span> <span class="token string">'a'</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>

        <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">!</span>stack<span class="token punctuation">.</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            res<span class="token punctuation">[</span><span class="token operator">--</span>index<span class="token punctuation">]</span> <span class="token operator">=</span> stack<span class="token punctuation">.</span><span class="token function">pop</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        <span class="token keyword">return</span> String<span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span>res<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        Solution solution <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Solution</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        String res <span class="token operator">=</span>solution<span class="token punctuation">.</span><span class="token function">removeDuplicateLetters</span><span class="token punctuation">(</span><span class="token string">"bcabc"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>res<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>Python 代码：</p>
<pre class="line-numbers language-python"><code class="language-python"><span class="token keyword">class</span> <span class="token class-name">Solution</span><span class="token punctuation">:</span>
    <span class="token keyword">def</span> <span class="token function">removeDuplicateLetters</span><span class="token punctuation">(</span>self<span class="token punctuation">,</span> s<span class="token punctuation">:</span> str<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">></span> str<span class="token punctuation">:</span>
        size <span class="token operator">=</span> len<span class="token punctuation">(</span>s<span class="token punctuation">)</span>
        stack <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span>

        <span class="token keyword">for</span> i <span class="token keyword">in</span> range<span class="token punctuation">(</span>size<span class="token punctuation">)</span><span class="token punctuation">:</span>
            <span class="token keyword">if</span> stack<span class="token punctuation">.</span>count<span class="token punctuation">(</span>s<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">:</span>
                <span class="token keyword">continue</span>
            <span class="token keyword">while</span> stack <span class="token operator">and</span> ord<span class="token punctuation">(</span>s<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token operator">&lt;</span> ord<span class="token punctuation">(</span>stack<span class="token punctuation">[</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">)</span> \
                    <span class="token operator">and</span> s<span class="token punctuation">.</span>find<span class="token punctuation">(</span>stack<span class="token punctuation">[</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">,</span> i<span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">:</span>
                stack<span class="token punctuation">.</span>pop<span class="token punctuation">(</span><span class="token punctuation">)</span>



            stack<span class="token punctuation">.</span>append<span class="token punctuation">(</span>s<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span>
        <span class="token keyword">return</span> <span class="token string">''</span><span class="token punctuation">.</span>join<span class="token punctuation">(</span>stack<span class="token punctuation">)</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
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